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Flow birefringence and conformational characteristics of molecules of para-and meta-isomers of polyoxyphenyl-benzoxazoleterephthalamides
Eur. Polym. J. Vol. 19, No. 9, pp. 841 846, 1983 Printed in Great Britain
0014-3057/83 $3.00+0.00 Pergamon Press Ltd
FLOW BIREFRINGENCE AND CONFORMATIONAL CHARACTERISTICS OF M O L E C U L E S OF PARAA N D M E T A - I S O M E R S OF P O L Y O X Y P H E N Y L BENZOXAZOLETEREPHTHALAMIDES V. N. TSVETKOV, N. V. POGODINA and L. V. STARCHENKO Institute of Macromolecular Compounds, Academy of Sciences of the U.S.S.R.. Bolshoi 31, Leningrad 199004, U.S.S.R.
(Received 21 October 1982) Abstract--Flow birefringence (FB) has been used to study sulphuric solutions of two homologous series of polyoxyphenylbenzoxazoleterephthalamides (POPhBT) differing in the position (para- or meta-) of phenyl ring in the chain. In the framework of the FB method alone by using the theory of flow birefringence for kinetically rigid wormlike chains, it was possible to determine quantitatively the optical anisotropy of the monomer unit Aa = (330 + 30) 10 25 cm 3 and the length of the Kuhn segment A = (330 + 30) A and A = (115 _+ 20) A for para- and meta-isomers, respectively. Analysis of possible mechanisms of flexibility in the chains of both polymers gives theoretical values of the rigidity parameter A in good agreement with experimental values of A, confirming the validity of the molecular models used.
sample 1), the dependence of viscosity on velocity gradient was measured. Figure 1 shows that the relative viscosity of solution r/r remains virtually constant over the range of velocity gradients g between 79 and 2.8 sec 1. This constancy indicates that no gradient dependence of intrinsic viscosity exists over the investigated range of shear rates. The values of intrinsic viscosities [r/] are given for the paraand recta-samples in Tables I and 2, respectively. To obtain the characteristic values of birefringencc
INTRODUCTION
C o p o l y m e r s of para- a n d meta-aromatic p o l y a m i d e s have been investigated previously by flow birefringence (FB), a n d the d e p e n d e n c e o f chain flexibility o f a r o m a t i c p o l y a m i d e s o n the para- a n d meta-positions o f phenyl rings in their chains has been s h o w n quantitatively [1]. T h e p r e s e n t w o r k is c o n c e r n e d with c o m p a r i s o n s o f c o n f o r m a t i o n a l characteristics of two h o m o l o g o u s series o f p o l y o x y p h e n y l b e n z o x a z o l e t e r e p h t h a l a m i d e s ( P O P h B T ) with different positions (para- or meta-) of the phenyl ring in the chain.
[n] = lim An,,'#C~lo, c-0 040
OH C~.N - N H - C O - - ~ CO-NH- ~ ~.O~
(P --p-- OPhBTI
OH N
(P --m--OPhBT) o
EXPERIMENTAL
The samples were obtained by low-temperature polycondensation [2] in solution of dimethylacetamide with lithium chloride. FB was measured with an instrument providing photoelectric recording of this effect [3, 4] in Teflon dynamooptimeters with an inner rotor. The rotor height along the path of the light beam was 4.4 and 1.5 cm and the gap width was 0.05 and 0.02 cm, respectively. Sulphuric acid (96%) with density p = 1.836g/cm 3, viscosity r/o = 0.22 g/cm.sec and refractive index n = 1.43 at wavelength 2 = 6300 A. at 21<'C was used as solvent. Viscometric data were obtained with an Ostwald capillary viscometer with efflux time of the solvent (H2SO4) = 67 sec. For a sample of the highest molecular weight (Table 1, 841
the values of An/gcqo were extrapolated to zero concentration (Fig. 2). The characteristic values of birefringence [n] and the values of reduced birefringence [n]i[_r/] = tim An/,qq0(r/,- 1) c~O 9~0
are listed in Tables 1 and 2. Particular attention was devoted to the high precision of measurements of orientation angles. Figure 3 shows the dependences of orientation angles 7` on the velocity gradient 9 at the minimum concentrations used. In all cases the linear part of the dependence 7` = 7,(9) was sufficiently large for a reliable determination of initial slopes of
842
V. N. TSVETKOV et al. Table 1. Dynamooptical characteristics of molecules of P - p - O P h B T in 96% sulphuric acid [ q ] " lO 2 (cm3/g)
No. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21
9.6 8.0 6.0 3.4 3.2 2.52 2.20 2.15 2.0 1.94 1.50 1.23 0.91 0.84 0.55 0.50 0.48 0.29 0.183 0.154 0.092
[n]-IO s (g 2 cm 4 sec 2)
± 0.2 ± 0.2 ± 0.2 __+0.1 ± 0.1 ± 0.07 ± 0.05 ± 0.03 __+0.1 ± 0.05 ± 0.05 ± 0.03 ± 0.01 ± 0.03 ± 0.03 ± 0.03 ± 0.03 ± 0.03 ± 0.003 ± 0.005 ± 0.002
[n]/[~/]" I0 ' ° (g i cm. sec 2)
4100 ± 200 3300 ± 100 2400 ± 70 1350 ± 50 1250 ± 50 920 ± 40 790 ± 30 770 ± 30 730 ± 30 680 ± 20 510 ± 20 410 ± 15 285 ± 10 235 ± 10 145 ± 5 115__+5 110 ± 5 61 ± 2 24 ± 1 14.6 ± 0.5 6.1 ± 0.2
430 ± 410 ± 400 ± 400 ± 390 ± 365 ± 360 ± 360 ± 365 ± 350 ± 340 ± 330 ± 310 ± 280 ± 260 ± 230± 230 ± 210 ± 130 ± 95 ± 66 ±
20 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 5 2
[Z/g]"
I05 (sec)
30.0 __+0.5 17.0 ± 0.5 11.4 ± 0.5 5.2 ± 0.2 5.0 ± 0.2 2.10 ± 0.05 1.80 ± 0.05 1.75 ± 0.05 1.7 _____0. l 1.59 ± 0.05 1.15 ± 0.05 0.85 ± 0.05 0.55 ± 0.05 0.40 ± 0.05 0.22 ± 0.01 0.175±0.005 0.165 ± 0.005 0.077 ± 0.003 0.030 ± 0.005 0.020 ± 0.005 0.010 ± 0.003
[n]/[Z/g]
M. I0 4
(g 2 cm'* sec)
(g/mol)
0.14 ± 0.02 0.19 ± 0.01 0.21 ± 0.02 0.26 ± 0.02 0.25 ± 0.02 0.44 ± 0.03 0.44 ± 0.03 0.44 ± 0.03 0.43 ± 0.04 0.43 __+0.03 0.45 ± 0.03 0.48 ± 0.05 0.56 ± 0.06 0.59 ± 0.08 0.64 ± 0. t5 0.66+0.12 0.67 + 0.12 0.79 ± 0.12 0.8 ± 0.2 0.7 ± 0.2 0.6 ± 0.4
5.5 3.7 3.4 2.7 2.7 1.5 1.4 1.4 1.5 1.4 1.3 1.2 1.05 0.83 0.64 0.60 0.59 0.40 0.22 0.20 0.17
± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± +
1.0 0.2 0.2 0.2 0.2 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.05 0.05 0.05 0.05 0.05 0.05 0.02 0.02 0.02
(Z/g)o~o. Concentration dependences for all samples are shown in Fig. 4, and the values of characteristic orientation angles are given in Tables 1 and 2.
~-,oF'%,o:(Q~,)
2.5
0.05
n
o.. I
z
o---2
RESULTS AND
DISCUSSION
2.0 c
,o-,-o
G
P--o 1.0
c
o
c
o
20
o
I
{"
40
60
It is k n o w n t h a t t h e c h a r a c t e r i s t i c o r i e n t a t i o n a n g l e
[Z/9], t h e intrinsic viscosity [t/] a n d t h e m o l e c u l a r weight M of t h e p o l y m e r a r e related by E q n (1) w h i c h is valid e x p e r i m e n t a l l y for a r o m a t i c r i g i d - c h a i n polym e r s at M > 104 [5, 6].
0--4 0-5
I
80
[q]~o
[Z/9] = G M - -
g sec -~
Fig. 1. Relative viscosity of solution q, = z/r 0 (z 0 and z are efflux times of the solvent and the solution at a concentration c) vs velocity gradient 9 for sample 1 in Table 1. N u m b e r s on lines correspond to concentrations: (1) c = 0.085; (2) c = 0.052; (3) c = 0.038; (4) c = 0.025; (5) c = 0.0129.10 2g/cm3. In the upper right-hand corner, the dependence (q~p/C),~o = (r/, - l/c),~ 0 on concentration is shown for the same sample.
(1)
RT
w h e r e R is t h e g a s c o n s t a n t , T is a b s o l u t e t e m p e r a ture. H e n c e , for s a m p l e s with M > 104, t h e v a l u e s of M were e v a l u a t e d f r o m E q n (1) by u s i n g G = 0.63 [7] for para-samples 1 18 (Table 1) a n d 0.5 for meta-sampies 1-11 (Table 2). T h e c h o i c e o f t h e v a l u e of G = 0.5 for meta-oxazoles is b a s e d o n t h e following, partly
Table 2. Dynamooptical characteristics of molecules of P - m - O P h B T in 96% sulphuric acid
No. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
[~/]" 102 (cm3/g) 3.7 3.2 2.9 2.70 2.45 2.25 2.15 1.45 1.37 1.30 1.23 0.50 0.40 0.28 0.151
_____0.2 ± 0.1 __-5__0.1 ± 0.05 ± 0.05 ± 0.05 ± 0.05 ± 0.03 ± 0.03 ± 0.03 ± 0.03 ± 0.03 ± 0.03 ± 0.03 ± 0.002
[n]" 10 a (g 2 cm 4 sec 2) 610 530 460 430 380 350 330 230 200 190 180 55 43 27 11.0
± 20 ± 20 ± 10 ± 10 ± 10 ± 10 ± 10 ± 10 ± 10 ± 10 ± 10 +__2 ± 2 ± 1 ± 0.2
[-n]/[r/]. 101° (g i cm. sec 2) 165 165 160 160 155 155 155 155 145 145 145 110 108 96 73
± ± ± ± ± ± ± ± ± ± ± ± ± + ±
5 5 5 5 5 5 5 5 5 5 5 5 5 5 2
[z/g]" 10s (sec) 6.9 5.5 4.5 3.4 3.1 2.9 2.6 1.20 1.00 0.93 0.83 0.18 0.13 0.070 0.025
± 0.3 __+0.3 ± 0.3 ± 0.1 ± 0.1 ± 0.1 ± 0.1 ± 0.05 ± 0.05 ± 0.05 ± 0.05 ± 0.02 ± 0.02 ± 0.005 ± 0.005
[n]/[Z/O ]
M" 10 - a
(g- z cm,~ sec)
(g/mol)
0.09 0.10 0.10 0.12 0.12 0.12 0.13 0.19 0.20 0.20 0.20 0.30 0.33 0.39 0.44
+ ± ± ± ± ± ± ± ± ± ± ± ± ± ±
0.01 0.01 0.0l 0.01 0.01 0.01 0.01 0.02 0.02 0.02 0.02 0.03 0.03 0.05 0.12
4.2 3.8 3.4 2.8 2.8 2.8 2.6 1.8 1.6 1.6 1.5 0.56 0.44 0.31 0.16
± ± ± ± ± ± ± ± ± ± ± ± ± ± ±
0.3 0.3 0.3 0.2 0.2 0.2 0.2 0.1 0.1 0.1 0.1 0.05 0.05 0.05 0.05
Flow' birefringence and characteristics of isomers of POPhBT
843
11
,,~,~:,..<:,__-o- ~ 4000 ~0-o--m-2
%
(a) 600, ,..~ °~ ~2
.,.~3 2ooo
•E
500 eo--- 7 ; 8
800
~E'.T:._
300 ~ c L
--
~9 250~--
<1 ~
II o--
,~--
~
5
4
400 ~
5
.______.o- 6
7
---tO----"
o. 13 o.o
r
120 ~ d b l ~
-
~ 1 4
~
o-15 0
80 ~F----o--o--o
o
20
r~ o ~)
o
0
9
io
~
o o
o o
0.02 i
18
17
c.lO z
I?
200
~ 19(g/cm 3) 50 o (~-.--2021 ~.o~:)~ o i o o 0.04 (I) i
~ O~o
9
°"-12 ~ 13"~14 c10~3 o o__i 15 ( g / c m )
0.05 i
O.I I
(I)
0.05
oJ
(2)
o.~
o.2
(2)
0.25 i
0.5 i
(3)
0.2 i
0.4 i
(5)
0.5
LO
(4)
0.5
1.0
(4)
l
i
i
i
Fig, 2. Values of An/gctlo vs concentration c. Numbers on the curves in Figs 2a, 3a and 4a correspond to sample numbers in Table 1. The abscissa (Fig. 2a) gives scale I for samples 1 3, scale 2 for samples 4 I 1 and 14 16, scale 3 for samples 12, 13, 17 and 18 and scale 4 for samples 19 21. Numbers on straight lines in Figs 2b, 3b and 4b are sample numbers in Table 2. The abscissa (Fig. 2b) gives scale 1 for samples I S. scale 2 for samples 9 II, scale 3 for samples 12 14 and scale 4 for sample 15.
g
SeC
-I
4000
2000'
(2) 6000 ( I )
8O00
4000
I
.
IO,0OO I
4000 2obo
'°I--~.~-%~
(3 ) (2)
g
sec
-I
(,)
-'';-,~-2"~" ,3
empirical, considerations. Limiting theoretical values of the coefficient G in the Gaussian range for an absolutely flexible-chain polymer (A = 20 A) and a kinetically rigid-chain polymer (A ~ 300A) are G = 0.1 [8] and G = 0.63 [7], respectively. The experimental value of the coefficient G for an aromatic polyamide of moderate (medium) rigidity (A - 50 AI like polymethaphenylene isophthalamide is (; = 0.32 [9]. It is clear that these three values of G satis~ the logarithmic dependence G = PIgA + Q which yields for meta-oxazoles at A ~ 120 A the value of G = 0.5. Comparison of molecular weights thus obtained and the corresponding intrinsic viscosities (Fig. 5) gives the dependences [r/] = 2 . 7 10 4 M1.4 for parasamples and [r/] = 1.3' 10 2 M0.95 for meta-samples. For samples with M < 104, molecular weights were determined according to the values of i-q] by using the M a r k K u h n relationships obtained. The molecular weights thus determined are given for all samples in Tables 1 and 2. The plot of the dependence [n]/[~l] = . / ( M ) is used for the quantitative determination of the parameters of chain anisotropy and rigidity. Points in Fig. 6 show the experimental data and solid curves show the theoretical dependences [10] determined according to the equation
Fig. 3. Orientation angles Z vs velocity gradient at the minimum concentration used. The abscissa in Fig. 3a gives scale I for samples 1 5 and scale 2 for samples 6.17. The abscissa in Fig. 3b gives scale 1 for samples 1 4 and 7, scale 2 for samples 8 11 and scale 3 for samples 12--15. Minimum concentrations ranged between 0.00l • 10 2 g/cm 3 for high molecular weight samples and 0.34.10 2g/cm3 for low molecular weight samples.
[,7] where h is the end-to-end distance of the polymer chain x = 2L/A is doubled n u m b e r of the K u h n segments in the chain B = 4n0~ 2 + 2)2/45kTn, Aa is the optical anisotropy of the m o n o m e r unit, S is the n u m b e r of m o n o m e r units in the K u h n segment and
V.N. TSVETKOV et al.
844
Io-
30
2
2 o ~ 15 ~ o o o . . ~
(a)
3
5=--" ~.
%
't
/I
/
4
~"
"
4
6
3 --
2
~
e
~
6
,
0,8~.,oL_
II
13
_ ,.
O.Ii':~ ~ ; 1 7
_19(g/cm~l
o.ool-
~
12
1
5
c'lO 2
(g/cm 3 )
0.11
14
0
°151
cr"T'°
0.5
I
t3)
0.05
0.05
0.1
(2)
0.2 *
0.4 (2) I
0.5
1.0 (3)
i
i
i
i
o.ol
0.1 ( I )
i
0.02 (I)
i
Fig. 4. Initial slopes (z/g)g~oof orientation angle vs concentration c. The abscissa in Fig. 4a gives scale 1 for samples 1-5, scale 2 for samples 6 17 and scale 3 for samples 18-21. The abscissa in Fig. 4b gives scale 1 for samples 1-7, scale 2 for samples 8-14 and scale 3 for sample 15.
(h 2) and (h 4) are the second a n d the fourth distribution m o m e n t s in h in an assembly of wormlike chains. Experimental data are in agreement with the shape of the theoretical curves at the values of S = 17 and Aa = 3 4 0 . 1 0 - 2 5 c m 3 for para-samples and S = 6.9 and Aa = 340" 10 -25 cm 3 for meta-samples. In this case the experimental values of reduced birefringence in the G a u s s i a n range ([n]/[t/])~ = BAaS = 460" 10 -10 g - 1 c m . s e c 2 for para-isomers a n d (In]/ [q])~ = 185"10-1° g - i c m ' s e c 2 for meta-isomers were used. Taking into account that for para-oxazole length of the identity period in the direction of the extended chain is ,i. = 19 A [-6], one obtains the length of the K u h n segment A = 2S = 320 A. F o r meta-oxazole the structural formula of the molecule gives )~ = 16.5 A and, correspondingly, A = 115 A. A n o t h e r m e t h o d for the determination of molecular parameters anisotropy a n d rigidity, is the plotting of the dependence [hi~lUg ] =f([n]/[q]) shown for para-oxazole in Fig. 7. According to the theory [10],
the points fit a straight line with the abscissa intercept equal to BAaS and the ordinate intercept equal to Bo(Aa/Mo) where B o = 8gNa(n 2 + 2)2/nqo and M 0 is the molecular weight of the m o n o m e r unit. In accord with this, by using the experimentally determined abscissa intercept 480' 1 0 - 1 ° g -1 c m . s e c 2 and ordinate intercept 1 . 5 g - 2 c m 4 s e c , one obtains A a = 320.10 -25 cm 3 and S = 19, which corresponds to the length of the K u h n segment A = 2S = 350 A. These results agree with the data obtained for para-oxazoles according to the molecular weight dependence of [n]/Dl] (Fig. 6). For meta-samples this plot has not been made because the range of the values of [n]/[q] is much narrower. For the quantitative discussion of these values of rigidity parameters of para- and meta-oxazoles, analy-
%
400
I
E
u 300
m
T~
8
E6 %4 -K" Ill
9
0 200
o~,3/"" "Y"
3
2
Ioo
I
I
I M'
2 IO - 4
I
I
I
I
3
4
5
6
Fig. 5. Intrinsic viscosity [r/] vs molecular weight M for the polymers plotted on a logarithmic scale. Line 1 (open circles)--para-samples; line 2 (filled circles) meta-samples.
0
I
I
I
I
I
I
2
3
4
5
M. lO-4 Fig. 6. Reduced birefringence [r/]/[q] vs molecular weight M for the polymers investigated. Solid curves are plotted according to the theory [10]. Curve 1 (open circles)--paraoxazoles; curve 2 (filled circles~-meta-oxazoles.
845
Flow birefringence and characteristics of isomers of POPhBT
the direction of the rotation axis by an angle ,',~ = 30 without any displacement. Hence. the rigidity' parameter, S 2, due to this mechanism is determined from Eqn (3) with the displacement 6:A = 0. Substitution of ,~ = 30 ~' gives $2 = 14.9"a 2. The deformation of the am)de group during thermal intramolecular chain motion causes a deviation of the structure of this group from coplanarity by' an angle q< The rigidity parameter $3 corresponding to this deformation mechanism of flexibility.' is given by.
1.5
[1] ~: ~
$3 = er2/sin2 ~ ' sin 2 0 2
0.5
I I00
(4}
where 0 is 6 0 and the angle q) is calculated from the equation U0(1 - cos c0) = 2RT (5) 200
300
400
Cn] c~-'~" ' lOi°(g-q cmsee2
500
)
Fig. 7. Dependence of [n]/[gjg] on [n]/[r/] for parasamples. The straight line is plotted by the least-squares method.
sis of mechanisms of flexibility in the chains of both polymers will be carried out. Possible structures and deformation mechanisms of flexibility of aromatic polyamide chains have been considered [1] and the additive action of these mechanisms has also been proved. The following flexibility mechanisms may be of major importance for the para-oxazole chains: two structural mechanisms (inequality of bond angles fi and ~. at nitrogen and carbon atoms of the am)de group and the presence of a heterocycle in the chain) and the third, deformation, mechanism (deviation from the coplanarity of bonds in the am)de group). The inequality of bond angles /~ and :~ leads to the %ending" of the para-oxazole chain. In this case, the advance along the polyamide chain by one am)de group is accompanied by a displacement of the rotation axis in the direction normal to it by a link 3 and by the rotation of the axis by an angle 0 = ,6 - z~. In accordance with this, each chain unit containing an am)de group and a phenyl ring may be replaced by two virtual mutually normal bonds A and 6, rotation being possible only about the former [11]. The rigidity parameter S. due to this mechanism is determined by the equation
At 293 K, the conjugation energy' of the am)de group U o = 88 kJ/mol [13] and the angle ~p is equal to 19' according to Eqn (5). Substitution of these values of 0 and q~ in Eqn (4) yields the rigidity' parameter S 3 = 48.9. a 2. All three above mechanisms of flexibility' described by Eqns (3) and (4) are typical not only' of para- but also of meta-oxazoles because both contain am)de groups and heterocycles. However, for the metaoxazole chain, the fourth principal mechanism should be taken into account: it is due to the fact the meta-oxazole contains an aromatic ring in the rectaposition. According to the foregoing considerations, the rigidity parameter is determined from Eqn (3) at 3 / A = 0 . 2 and O 60 and is equal to $ 4 = 3.3' 0.2 . As already shown [1], the resulting flexibility, of the polyamide chain is a sum of flexibilities due to different mechanisms. Noting that each identity period of oxazole chain contains two am)de groups, the resultant flexibility.' I/S is expressed by' Eqns (61 and (7) for para- and meta-samples, respectively 1
2
1
theor
1
2
+s3 2
1
2
1
(7)
s .....
theor
where $1, $2 and S,, are determined according to Eqn. (3) and $3 is determined according to Eqn (4). Substitution of the above values of S~. $2 and $3 into Eqn (6) gives Sp~,~ = 7.7.0. 2 at fi - ~ = 12 ° and theor
S1 = 0 - 2
+
1
cosO],"\
2 + Aslny)
(3)
where 02 is the coefficient characterizing the hindrance to rotation about the C and N phenyl bonds, the parameter b/A = 0.2 for the polyamide chain and the possible difference between the bond angles fl in the amide group ranges between 6 and 1 2 [12]. Substitution of these values of 6/A and ,8 - :~ into Eqn {3) gives the limiting values of the rigidity parameter S~ = 357-er 2 and S~ = 87.8'er 2 for the minimum and the maximum difference in bond angles, respectively. The presence of a heterocycle in the chain changes
Sm~ = 8.8" ere at fi - :~ = 6% This difference in the theor
values of Sp.,~ is virtually within experimental error theor
gp,~ = 8.3"0.-'. Iheor
Similarly taking into account S,~, one obtains from Eqn (7) the value of Smut, = 2.4' 0.2. Comparison of theor
these values, on the assumption that the coefficients 0. for para- and meta-isomers are equal, shows that the rigidity parameters S differ by a factor of about 3: Sp~,,//S,,,,,, = 3.5. This result is in reasonable agreetheor theor
ment with the experimental evaluation of rigidity,
846
V.N. TSVETKOV et al.
of oxazole chains according to the FB data. Sp,r,/S,,,,t, = 2,6, which confirms the validity of the exp
exp
molecular model used. C o m p a r i s o n of experimental and theoretical values of the rigidity parameter gp,r, = Sv,ra gives the value exp
theor
of the parameter of hindrance to rotation for the para-oxazole chain awr a = 1.5. A similar value of a,,,,ta = 1.7 is also obtained for the meta-oxazole chain. This fact justifies the assumption that hindrance coefficients a are equal which was used above, for the comparison of rigidities of para- and meta-isomers. These values of hindrance parameters are much lower than the corresponding values for flexible-chain polymers as repeatedly reported previously.
REFERENCES
I. V. N. Tsvetkov, N. V. Pogodina, L. V. Starchenko, B. F. Malichenko, O. N. Tsypina and T. A. Kulichikhina, Vvsokomolek. Soedin. 23A, 26 (1981).
2. V. N. Kolot, G. J. Kudriavtsev and G. D. Litovchenko, Vysokomolek. Soedin. 20A, 546 (1978). 3. V. N. Tsvetkov, V. E. Eskin and S. Ya Frenkel, Struktura Makromolecul v Rastvorakh. Nauka, Moscow (1964). 4. S. N. Penkov and V. Z. Stepanenko, Opt. Spektrosk. 14, 156 (1963). 5. N. V. Pogodina, L. V. Starchenko, K. S. Pozhivilko, V. D. Kalmykova, T. A. Kulichikhina, A. V. Volokhina, V. I. Kudriavtsev and V. N. Tsvethov, Vysokomolek. Soedin 23A, 2185 (1981). 6. I. N. Shtennikova, T. V. Peker, T. I. Garmanova, G. F. Kolbina, L. V. Avrorova, A. V. Tokarev, V. I. Kudriavtsev and V. N. Tsvetkov, Vysokomolek. Soedin 23A, 2510 (1981). 7. Ch. Chaeffey, J. chim. Phys. 63, 137 (1966). 8. B. H. J. Zimm, J. chem. Phys. 24, 269 (1956). 9. V. N. Tsvetkov, E. N. Zakharova and N. A. Mikhailova, Dokl. Acad. Nauk SSSR 224, 1365 (1975). 10. V. N. Tsvetkov, Dokl. Akad. Nauk SSSR 266, 670 (1982). 11. H. Benoit, J. Polym. Sci. 3, 376 (1948). 12. U. Schmueli, W. Traub and K. Rosenheck, J. Polym. Sci. A-2, 3, 515 (1969). 13. G. Pauling, The Nature of Chemical Bond, p. 142 (1947).
0014-3057/83 $3.00+0.00 Pergamon Press Ltd
FLOW BIREFRINGENCE AND CONFORMATIONAL CHARACTERISTICS OF M O L E C U L E S OF PARAA N D M E T A - I S O M E R S OF P O L Y O X Y P H E N Y L BENZOXAZOLETEREPHTHALAMIDES V. N. TSVETKOV, N. V. POGODINA and L. V. STARCHENKO Institute of Macromolecular Compounds, Academy of Sciences of the U.S.S.R.. Bolshoi 31, Leningrad 199004, U.S.S.R.
(Received 21 October 1982) Abstract--Flow birefringence (FB) has been used to study sulphuric solutions of two homologous series of polyoxyphenylbenzoxazoleterephthalamides (POPhBT) differing in the position (para- or meta-) of phenyl ring in the chain. In the framework of the FB method alone by using the theory of flow birefringence for kinetically rigid wormlike chains, it was possible to determine quantitatively the optical anisotropy of the monomer unit Aa = (330 + 30) 10 25 cm 3 and the length of the Kuhn segment A = (330 + 30) A and A = (115 _+ 20) A for para- and meta-isomers, respectively. Analysis of possible mechanisms of flexibility in the chains of both polymers gives theoretical values of the rigidity parameter A in good agreement with experimental values of A, confirming the validity of the molecular models used.
sample 1), the dependence of viscosity on velocity gradient was measured. Figure 1 shows that the relative viscosity of solution r/r remains virtually constant over the range of velocity gradients g between 79 and 2.8 sec 1. This constancy indicates that no gradient dependence of intrinsic viscosity exists over the investigated range of shear rates. The values of intrinsic viscosities [r/] are given for the paraand recta-samples in Tables I and 2, respectively. To obtain the characteristic values of birefringencc
INTRODUCTION
C o p o l y m e r s of para- a n d meta-aromatic p o l y a m i d e s have been investigated previously by flow birefringence (FB), a n d the d e p e n d e n c e o f chain flexibility o f a r o m a t i c p o l y a m i d e s o n the para- a n d meta-positions o f phenyl rings in their chains has been s h o w n quantitatively [1]. T h e p r e s e n t w o r k is c o n c e r n e d with c o m p a r i s o n s o f c o n f o r m a t i o n a l characteristics of two h o m o l o g o u s series o f p o l y o x y p h e n y l b e n z o x a z o l e t e r e p h t h a l a m i d e s ( P O P h B T ) with different positions (para- or meta-) of the phenyl ring in the chain.
[n] = lim An,,'#C~lo, c-0 040
OH C~.N - N H - C O - - ~ CO-NH- ~ ~.O~
(P --p-- OPhBTI
OH N
(P --m--OPhBT) o
EXPERIMENTAL
The samples were obtained by low-temperature polycondensation [2] in solution of dimethylacetamide with lithium chloride. FB was measured with an instrument providing photoelectric recording of this effect [3, 4] in Teflon dynamooptimeters with an inner rotor. The rotor height along the path of the light beam was 4.4 and 1.5 cm and the gap width was 0.05 and 0.02 cm, respectively. Sulphuric acid (96%) with density p = 1.836g/cm 3, viscosity r/o = 0.22 g/cm.sec and refractive index n = 1.43 at wavelength 2 = 6300 A. at 21<'C was used as solvent. Viscometric data were obtained with an Ostwald capillary viscometer with efflux time of the solvent (H2SO4) = 67 sec. For a sample of the highest molecular weight (Table 1, 841
the values of An/gcqo were extrapolated to zero concentration (Fig. 2). The characteristic values of birefringence [n] and the values of reduced birefringence [n]i[_r/] = tim An/,qq0(r/,- 1) c~O 9~0
are listed in Tables 1 and 2. Particular attention was devoted to the high precision of measurements of orientation angles. Figure 3 shows the dependences of orientation angles 7` on the velocity gradient 9 at the minimum concentrations used. In all cases the linear part of the dependence 7` = 7,(9) was sufficiently large for a reliable determination of initial slopes of
842
V. N. TSVETKOV et al. Table 1. Dynamooptical characteristics of molecules of P - p - O P h B T in 96% sulphuric acid [ q ] " lO 2 (cm3/g)
No. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21
9.6 8.0 6.0 3.4 3.2 2.52 2.20 2.15 2.0 1.94 1.50 1.23 0.91 0.84 0.55 0.50 0.48 0.29 0.183 0.154 0.092
[n]-IO s (g 2 cm 4 sec 2)
± 0.2 ± 0.2 ± 0.2 __+0.1 ± 0.1 ± 0.07 ± 0.05 ± 0.03 __+0.1 ± 0.05 ± 0.05 ± 0.03 ± 0.01 ± 0.03 ± 0.03 ± 0.03 ± 0.03 ± 0.03 ± 0.003 ± 0.005 ± 0.002
[n]/[~/]" I0 ' ° (g i cm. sec 2)
4100 ± 200 3300 ± 100 2400 ± 70 1350 ± 50 1250 ± 50 920 ± 40 790 ± 30 770 ± 30 730 ± 30 680 ± 20 510 ± 20 410 ± 15 285 ± 10 235 ± 10 145 ± 5 115__+5 110 ± 5 61 ± 2 24 ± 1 14.6 ± 0.5 6.1 ± 0.2
430 ± 410 ± 400 ± 400 ± 390 ± 365 ± 360 ± 360 ± 365 ± 350 ± 340 ± 330 ± 310 ± 280 ± 260 ± 230± 230 ± 210 ± 130 ± 95 ± 66 ±
20 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 5 2
[Z/g]"
I05 (sec)
30.0 __+0.5 17.0 ± 0.5 11.4 ± 0.5 5.2 ± 0.2 5.0 ± 0.2 2.10 ± 0.05 1.80 ± 0.05 1.75 ± 0.05 1.7 _____0. l 1.59 ± 0.05 1.15 ± 0.05 0.85 ± 0.05 0.55 ± 0.05 0.40 ± 0.05 0.22 ± 0.01 0.175±0.005 0.165 ± 0.005 0.077 ± 0.003 0.030 ± 0.005 0.020 ± 0.005 0.010 ± 0.003
[n]/[Z/g]
M. I0 4
(g 2 cm'* sec)
(g/mol)
0.14 ± 0.02 0.19 ± 0.01 0.21 ± 0.02 0.26 ± 0.02 0.25 ± 0.02 0.44 ± 0.03 0.44 ± 0.03 0.44 ± 0.03 0.43 ± 0.04 0.43 __+0.03 0.45 ± 0.03 0.48 ± 0.05 0.56 ± 0.06 0.59 ± 0.08 0.64 ± 0. t5 0.66+0.12 0.67 + 0.12 0.79 ± 0.12 0.8 ± 0.2 0.7 ± 0.2 0.6 ± 0.4
5.5 3.7 3.4 2.7 2.7 1.5 1.4 1.4 1.5 1.4 1.3 1.2 1.05 0.83 0.64 0.60 0.59 0.40 0.22 0.20 0.17
± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± +
1.0 0.2 0.2 0.2 0.2 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.05 0.05 0.05 0.05 0.05 0.05 0.02 0.02 0.02
(Z/g)o~o. Concentration dependences for all samples are shown in Fig. 4, and the values of characteristic orientation angles are given in Tables 1 and 2.
~-,oF'%,o:(Q~,)
2.5
0.05
n
o.. I
z
o---2
RESULTS AND
DISCUSSION
2.0 c
,o-,-o
G
P--o 1.0
c
o
c
o
20
o
I
{"
40
60
It is k n o w n t h a t t h e c h a r a c t e r i s t i c o r i e n t a t i o n a n g l e
[Z/9], t h e intrinsic viscosity [t/] a n d t h e m o l e c u l a r weight M of t h e p o l y m e r a r e related by E q n (1) w h i c h is valid e x p e r i m e n t a l l y for a r o m a t i c r i g i d - c h a i n polym e r s at M > 104 [5, 6].
0--4 0-5
I
80
[q]~o
[Z/9] = G M - -
g sec -~
Fig. 1. Relative viscosity of solution q, = z/r 0 (z 0 and z are efflux times of the solvent and the solution at a concentration c) vs velocity gradient 9 for sample 1 in Table 1. N u m b e r s on lines correspond to concentrations: (1) c = 0.085; (2) c = 0.052; (3) c = 0.038; (4) c = 0.025; (5) c = 0.0129.10 2g/cm3. In the upper right-hand corner, the dependence (q~p/C),~o = (r/, - l/c),~ 0 on concentration is shown for the same sample.
(1)
RT
w h e r e R is t h e g a s c o n s t a n t , T is a b s o l u t e t e m p e r a ture. H e n c e , for s a m p l e s with M > 104, t h e v a l u e s of M were e v a l u a t e d f r o m E q n (1) by u s i n g G = 0.63 [7] for para-samples 1 18 (Table 1) a n d 0.5 for meta-sampies 1-11 (Table 2). T h e c h o i c e o f t h e v a l u e of G = 0.5 for meta-oxazoles is b a s e d o n t h e following, partly
Table 2. Dynamooptical characteristics of molecules of P - m - O P h B T in 96% sulphuric acid
No. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
[~/]" 102 (cm3/g) 3.7 3.2 2.9 2.70 2.45 2.25 2.15 1.45 1.37 1.30 1.23 0.50 0.40 0.28 0.151
_____0.2 ± 0.1 __-5__0.1 ± 0.05 ± 0.05 ± 0.05 ± 0.05 ± 0.03 ± 0.03 ± 0.03 ± 0.03 ± 0.03 ± 0.03 ± 0.03 ± 0.002
[n]" 10 a (g 2 cm 4 sec 2) 610 530 460 430 380 350 330 230 200 190 180 55 43 27 11.0
± 20 ± 20 ± 10 ± 10 ± 10 ± 10 ± 10 ± 10 ± 10 ± 10 ± 10 +__2 ± 2 ± 1 ± 0.2
[-n]/[r/]. 101° (g i cm. sec 2) 165 165 160 160 155 155 155 155 145 145 145 110 108 96 73
± ± ± ± ± ± ± ± ± ± ± ± ± + ±
5 5 5 5 5 5 5 5 5 5 5 5 5 5 2
[z/g]" 10s (sec) 6.9 5.5 4.5 3.4 3.1 2.9 2.6 1.20 1.00 0.93 0.83 0.18 0.13 0.070 0.025
± 0.3 __+0.3 ± 0.3 ± 0.1 ± 0.1 ± 0.1 ± 0.1 ± 0.05 ± 0.05 ± 0.05 ± 0.05 ± 0.02 ± 0.02 ± 0.005 ± 0.005
[n]/[Z/O ]
M" 10 - a
(g- z cm,~ sec)
(g/mol)
0.09 0.10 0.10 0.12 0.12 0.12 0.13 0.19 0.20 0.20 0.20 0.30 0.33 0.39 0.44
+ ± ± ± ± ± ± ± ± ± ± ± ± ± ±
0.01 0.01 0.0l 0.01 0.01 0.01 0.01 0.02 0.02 0.02 0.02 0.03 0.03 0.05 0.12
4.2 3.8 3.4 2.8 2.8 2.8 2.6 1.8 1.6 1.6 1.5 0.56 0.44 0.31 0.16
± ± ± ± ± ± ± ± ± ± ± ± ± ± ±
0.3 0.3 0.3 0.2 0.2 0.2 0.2 0.1 0.1 0.1 0.1 0.05 0.05 0.05 0.05
Flow' birefringence and characteristics of isomers of POPhBT
843
11
,,~,~:,..<:,__-o- ~ 4000 ~0-o--m-2
%
(a) 600, ,..~ °~ ~2
.,.~3 2ooo
•E
500 eo--- 7 ; 8
800
~E'.T:._
300 ~ c L
--
~9 250~--
<1 ~
II o--
,~--
~
5
4
400 ~
5
.______.o- 6
7
---tO----"
o. 13 o.o
r
120 ~ d b l ~
-
~ 1 4
~
o-15 0
80 ~F----o--o--o
o
20
r~ o ~)
o
0
9
io
~
o o
o o
0.02 i
18
17
c.lO z
I?
200
~ 19(g/cm 3) 50 o (~-.--2021 ~.o~:)~ o i o o 0.04 (I) i
~ O~o
9
°"-12 ~ 13"~14 c10~3 o o__i 15 ( g / c m )
0.05 i
O.I I
(I)
0.05
oJ
(2)
o.~
o.2
(2)
0.25 i
0.5 i
(3)
0.2 i
0.4 i
(5)
0.5
LO
(4)
0.5
1.0
(4)
l
i
i
i
Fig, 2. Values of An/gctlo vs concentration c. Numbers on the curves in Figs 2a, 3a and 4a correspond to sample numbers in Table 1. The abscissa (Fig. 2a) gives scale I for samples 1 3, scale 2 for samples 4 I 1 and 14 16, scale 3 for samples 12, 13, 17 and 18 and scale 4 for samples 19 21. Numbers on straight lines in Figs 2b, 3b and 4b are sample numbers in Table 2. The abscissa (Fig. 2b) gives scale 1 for samples I S. scale 2 for samples 9 II, scale 3 for samples 12 14 and scale 4 for sample 15.
g
SeC
-I
4000
2000'
(2) 6000 ( I )
8O00
4000
I
.
IO,0OO I
4000 2obo
'°I--~.~-%~
(3 ) (2)
g
sec
-I
(,)
-'';-,~-2"~" ,3
empirical, considerations. Limiting theoretical values of the coefficient G in the Gaussian range for an absolutely flexible-chain polymer (A = 20 A) and a kinetically rigid-chain polymer (A ~ 300A) are G = 0.1 [8] and G = 0.63 [7], respectively. The experimental value of the coefficient G for an aromatic polyamide of moderate (medium) rigidity (A - 50 AI like polymethaphenylene isophthalamide is (; = 0.32 [9]. It is clear that these three values of G satis~ the logarithmic dependence G = PIgA + Q which yields for meta-oxazoles at A ~ 120 A the value of G = 0.5. Comparison of molecular weights thus obtained and the corresponding intrinsic viscosities (Fig. 5) gives the dependences [r/] = 2 . 7 10 4 M1.4 for parasamples and [r/] = 1.3' 10 2 M0.95 for meta-samples. For samples with M < 104, molecular weights were determined according to the values of i-q] by using the M a r k K u h n relationships obtained. The molecular weights thus determined are given for all samples in Tables 1 and 2. The plot of the dependence [n]/[~l] = . / ( M ) is used for the quantitative determination of the parameters of chain anisotropy and rigidity. Points in Fig. 6 show the experimental data and solid curves show the theoretical dependences [10] determined according to the equation
[,7] where h is the end-to-end distance of the polymer chain x = 2L/A is doubled n u m b e r of the K u h n segments in the chain B = 4n0~ 2 + 2)2/45kTn, Aa is the optical anisotropy of the m o n o m e r unit, S is the n u m b e r of m o n o m e r units in the K u h n segment and
V.N. TSVETKOV et al.
844
Io-
30
2
2 o ~ 15 ~ o o o . . ~
(a)
3
5=--" ~.
%
't
/I
/
4
~"
"
4
6
3 --
2
~
e
~
6
,
0,8~.,oL_
II
13
_ ,.
O.Ii':~ ~ ; 1 7
_19(g/cm~l
o.ool-
~
12
1
5
c'lO 2
(g/cm 3 )
0.11
14
0
°151
cr"T'°
0.5
I
t3)
0.05
0.05
0.1
(2)
0.2 *
0.4 (2) I
0.5
1.0 (3)
i
i
i
i
o.ol
0.1 ( I )
i
0.02 (I)
i
Fig. 4. Initial slopes (z/g)g~oof orientation angle vs concentration c. The abscissa in Fig. 4a gives scale 1 for samples 1-5, scale 2 for samples 6 17 and scale 3 for samples 18-21. The abscissa in Fig. 4b gives scale 1 for samples 1-7, scale 2 for samples 8-14 and scale 3 for sample 15.
(h 2) and (h 4) are the second a n d the fourth distribution m o m e n t s in h in an assembly of wormlike chains. Experimental data are in agreement with the shape of the theoretical curves at the values of S = 17 and Aa = 3 4 0 . 1 0 - 2 5 c m 3 for para-samples and S = 6.9 and Aa = 340" 10 -25 cm 3 for meta-samples. In this case the experimental values of reduced birefringence in the G a u s s i a n range ([n]/[t/])~ = BAaS = 460" 10 -10 g - 1 c m . s e c 2 for para-isomers a n d (In]/ [q])~ = 185"10-1° g - i c m ' s e c 2 for meta-isomers were used. Taking into account that for para-oxazole length of the identity period in the direction of the extended chain is ,i. = 19 A [-6], one obtains the length of the K u h n segment A = 2S = 320 A. F o r meta-oxazole the structural formula of the molecule gives )~ = 16.5 A and, correspondingly, A = 115 A. A n o t h e r m e t h o d for the determination of molecular parameters anisotropy a n d rigidity, is the plotting of the dependence [hi~lUg ] =f([n]/[q]) shown for para-oxazole in Fig. 7. According to the theory [10],
the points fit a straight line with the abscissa intercept equal to BAaS and the ordinate intercept equal to Bo(Aa/Mo) where B o = 8gNa(n 2 + 2)2/nqo and M 0 is the molecular weight of the m o n o m e r unit. In accord with this, by using the experimentally determined abscissa intercept 480' 1 0 - 1 ° g -1 c m . s e c 2 and ordinate intercept 1 . 5 g - 2 c m 4 s e c , one obtains A a = 320.10 -25 cm 3 and S = 19, which corresponds to the length of the K u h n segment A = 2S = 350 A. These results agree with the data obtained for para-oxazoles according to the molecular weight dependence of [n]/Dl] (Fig. 6). For meta-samples this plot has not been made because the range of the values of [n]/[q] is much narrower. For the quantitative discussion of these values of rigidity parameters of para- and meta-oxazoles, analy-
%
400
I
E
u 300
m
T~
8
E6 %4 -K" Ill
9
0 200
o~,3/"" "Y"
3
2
Ioo
I
I
I M'
2 IO - 4
I
I
I
I
3
4
5
6
Fig. 5. Intrinsic viscosity [r/] vs molecular weight M for the polymers plotted on a logarithmic scale. Line 1 (open circles)--para-samples; line 2 (filled circles) meta-samples.
0
I
I
I
I
I
I
2
3
4
5
M. lO-4 Fig. 6. Reduced birefringence [r/]/[q] vs molecular weight M for the polymers investigated. Solid curves are plotted according to the theory [10]. Curve 1 (open circles)--paraoxazoles; curve 2 (filled circles~-meta-oxazoles.
845
Flow birefringence and characteristics of isomers of POPhBT
the direction of the rotation axis by an angle ,',~ = 30 without any displacement. Hence. the rigidity' parameter, S 2, due to this mechanism is determined from Eqn (3) with the displacement 6:A = 0. Substitution of ,~ = 30 ~' gives $2 = 14.9"a 2. The deformation of the am)de group during thermal intramolecular chain motion causes a deviation of the structure of this group from coplanarity by' an angle q< The rigidity parameter $3 corresponding to this deformation mechanism of flexibility.' is given by.
1.5
[1] ~: ~
$3 = er2/sin2 ~ ' sin 2 0 2
0.5
I I00
(4}
where 0 is 6 0 and the angle q) is calculated from the equation U0(1 - cos c0) = 2RT (5) 200
300
400
Cn] c~-'~" ' lOi°(g-q cmsee2
500
)
Fig. 7. Dependence of [n]/[gjg] on [n]/[r/] for parasamples. The straight line is plotted by the least-squares method.
sis of mechanisms of flexibility in the chains of both polymers will be carried out. Possible structures and deformation mechanisms of flexibility of aromatic polyamide chains have been considered [1] and the additive action of these mechanisms has also been proved. The following flexibility mechanisms may be of major importance for the para-oxazole chains: two structural mechanisms (inequality of bond angles fi and ~. at nitrogen and carbon atoms of the am)de group and the presence of a heterocycle in the chain) and the third, deformation, mechanism (deviation from the coplanarity of bonds in the am)de group). The inequality of bond angles /~ and :~ leads to the %ending" of the para-oxazole chain. In this case, the advance along the polyamide chain by one am)de group is accompanied by a displacement of the rotation axis in the direction normal to it by a link 3 and by the rotation of the axis by an angle 0 = ,6 - z~. In accordance with this, each chain unit containing an am)de group and a phenyl ring may be replaced by two virtual mutually normal bonds A and 6, rotation being possible only about the former [11]. The rigidity parameter S. due to this mechanism is determined by the equation
At 293 K, the conjugation energy' of the am)de group U o = 88 kJ/mol [13] and the angle ~p is equal to 19' according to Eqn (5). Substitution of these values of 0 and q~ in Eqn (4) yields the rigidity' parameter S 3 = 48.9. a 2. All three above mechanisms of flexibility' described by Eqns (3) and (4) are typical not only' of para- but also of meta-oxazoles because both contain am)de groups and heterocycles. However, for the metaoxazole chain, the fourth principal mechanism should be taken into account: it is due to the fact the meta-oxazole contains an aromatic ring in the rectaposition. According to the foregoing considerations, the rigidity parameter is determined from Eqn (3) at 3 / A = 0 . 2 and O 60 and is equal to $ 4 = 3.3' 0.2 . As already shown [1], the resulting flexibility, of the polyamide chain is a sum of flexibilities due to different mechanisms. Noting that each identity period of oxazole chain contains two am)de groups, the resultant flexibility.' I/S is expressed by' Eqns (61 and (7) for para- and meta-samples, respectively 1
2
1
theor
1
2
+s3 2
1
2
1
(7)
s .....
theor
where $1, $2 and S,, are determined according to Eqn. (3) and $3 is determined according to Eqn (4). Substitution of the above values of S~. $2 and $3 into Eqn (6) gives Sp~,~ = 7.7.0. 2 at fi - ~ = 12 ° and theor
S1 = 0 - 2
+
1
cosO],"\
2 + Aslny)
(3)
where 02 is the coefficient characterizing the hindrance to rotation about the C and N phenyl bonds, the parameter b/A = 0.2 for the polyamide chain and the possible difference between the bond angles fl in the amide group ranges between 6 and 1 2 [12]. Substitution of these values of 6/A and ,8 - :~ into Eqn {3) gives the limiting values of the rigidity parameter S~ = 357-er 2 and S~ = 87.8'er 2 for the minimum and the maximum difference in bond angles, respectively. The presence of a heterocycle in the chain changes
Sm~ = 8.8" ere at fi - :~ = 6% This difference in the theor
values of Sp.,~ is virtually within experimental error theor
gp,~ = 8.3"0.-'. Iheor
Similarly taking into account S,~, one obtains from Eqn (7) the value of Smut, = 2.4' 0.2. Comparison of theor
these values, on the assumption that the coefficients 0. for para- and meta-isomers are equal, shows that the rigidity parameters S differ by a factor of about 3: Sp~,,//S,,,,,, = 3.5. This result is in reasonable agreetheor theor
ment with the experimental evaluation of rigidity,
846
V.N. TSVETKOV et al.
of oxazole chains according to the FB data. Sp,r,/S,,,,t, = 2,6, which confirms the validity of the exp
exp
molecular model used. C o m p a r i s o n of experimental and theoretical values of the rigidity parameter gp,r, = Sv,ra gives the value exp
theor
of the parameter of hindrance to rotation for the para-oxazole chain awr a = 1.5. A similar value of a,,,,ta = 1.7 is also obtained for the meta-oxazole chain. This fact justifies the assumption that hindrance coefficients a are equal which was used above, for the comparison of rigidities of para- and meta-isomers. These values of hindrance parameters are much lower than the corresponding values for flexible-chain polymers as repeatedly reported previously.
REFERENCES
I. V. N. Tsvetkov, N. V. Pogodina, L. V. Starchenko, B. F. Malichenko, O. N. Tsypina and T. A. Kulichikhina, Vvsokomolek. Soedin. 23A, 26 (1981).
2. V. N. Kolot, G. J. Kudriavtsev and G. D. Litovchenko, Vysokomolek. Soedin. 20A, 546 (1978). 3. V. N. Tsvetkov, V. E. Eskin and S. Ya Frenkel, Struktura Makromolecul v Rastvorakh. Nauka, Moscow (1964). 4. S. N. Penkov and V. Z. Stepanenko, Opt. Spektrosk. 14, 156 (1963). 5. N. V. Pogodina, L. V. Starchenko, K. S. Pozhivilko, V. D. Kalmykova, T. A. Kulichikhina, A. V. Volokhina, V. I. Kudriavtsev and V. N. Tsvethov, Vysokomolek. Soedin 23A, 2185 (1981). 6. I. N. Shtennikova, T. V. Peker, T. I. Garmanova, G. F. Kolbina, L. V. Avrorova, A. V. Tokarev, V. I. Kudriavtsev and V. N. Tsvetkov, Vysokomolek. Soedin 23A, 2510 (1981). 7. Ch. Chaeffey, J. chim. Phys. 63, 137 (1966). 8. B. H. J. Zimm, J. chem. Phys. 24, 269 (1956). 9. V. N. Tsvetkov, E. N. Zakharova and N. A. Mikhailova, Dokl. Acad. Nauk SSSR 224, 1365 (1975). 10. V. N. Tsvetkov, Dokl. Akad. Nauk SSSR 266, 670 (1982). 11. H. Benoit, J. Polym. Sci. 3, 376 (1948). 12. U. Schmueli, W. Traub and K. Rosenheck, J. Polym. Sci. A-2, 3, 515 (1969). 13. G. Pauling, The Nature of Chemical Bond, p. 142 (1947).